The instant invention concerns a transparent substrate provided with a thin-film stack comprising at least one metallic film with reflective properties in the infrared, under which is disposed a first antireflection coating consisting essentially of one or more metal oxides, and an upper protective metallic film surmounted in turn by a second upper antireflection coating.
It is known that substrates provided with a film stack comprising silver as a functional layer can be relatively sensitive to chemical stresses. To protect the silver film against attack by oxygen, the silver films are therefore generally protected by a metallic protective film which is applied over the silver film and has a greater affinity for oxygen. In addition, the subsequent antireflection coating is of essential importance to the chemical resistance and mechanical strength of the stack of films. Various multilayer film structures have been proposed for the antireflection coating in order to improve the chemical resistance and the mechanical strength of such stacks.
A film stack of this type, possessing high mechanical strength and high chemical resistance, is known from Document EP 0 593 883 B1. In this known film stack, a coating comprising three nonmetallic films is applied over the metallic protective film and, in this coating, the nonmetallic film adjacent the metallic protective film contains the metal of this metallic protective film, not as an essential constituent of its chemical composition. In the case concerned, the nonmetallic coating preferably consists of alternating zinc oxide and titanium oxide films applied by the reactive cathode sputtering process. It is assumed in this case that a zinc titanate film in the subnanometer range forms at each interface, thereby reinforcing the protective effect of the metallic protective film. In the case under consideration, a final, nonmetallic film, consisting in particular of titanium oxide, is preferably applied on top of the nonmetallic coating.
Such a film stack that includes titanium oxide films is relatively costly to manufacture, since such films can be produced only at a relatively low sputtering rate. Moreover, to apply a total of four extra films over the metallic protective film in a continuous-feed coating installation, it is necessary to have at least four extra sputtering stations, which are not always present in existing coating plants.